1. Field
This disclosure relates to optical modulators for application in fiber optic communications systems and other electro-optic systems.
2. Description of the Related Art
Optical modulators are essential components for fiber optic communication, RF photonics, instrumentation and optical signal processing applications. Desired characteristics of optical modulators are a modulation bandwidth as high as possible while maintaining a drive voltage as low as possible. Usually achieving these two properties requires conflicting sets of design rules. For example ultra-low drive voltage can be achieved using substrate removed (SURE) waveguides (JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli, “0.3 V drive GaAs/AlGaAs substrate removed Mach-Zehnder intensity modulators,” Appl. Phys. Letters, vol. 92, 201103, 2008). SURE waveguides are formed in a semiconductor epitaxial layer that is removed from its original substrate. Both sides of the epitaxial layer can be processed, enabling very novel designs. Such submicron thick waveguides have very high vertical index contrast and can guide the optical wave with very low loss of about a few dB/cm (JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli, “Propagation loss study of very compact GaAs/AlGaAs substrate removed waveguides,” Optics Express, Vol. 171, No. 5, 2009). However metal electrodes cannot be used on such waveguides since overlap of the optical mode with the metal typically results in excessive optical propagation loss. This difficulty is circumvented using doped semiconducting layers as buried electrodes. Therefore very strong electric fields overlapping very well with optical mode in the waveguide can be generated. resulting in ultra-low drive voltage. However finite sheet resistance of the doped semiconductor layers may create excessive resistance and electrode loss which in turn limits the bandwidth of the modulator to about 30 GHz (Selim Dogru, JaeHyuk Shin and Nadir Dagli, “Wide Bandwidth Design of Ultra-Low Voltage Substrate-Removed Electro-Optic Mach-Zehnder Intensity Modulators,” Integrated Photonics and Nanophotonics Research Conference Proceedings, Paper IWA6, Honolulu, Hi., Jul. 12-17, 2009; Selim Dogru, Jae Hyuk Shin and Nadir Dagli, “Traveling Wave Electrodes for Substrate Removed Electro-optic Modulators with Buried Doped Semiconductor Electrodes” IEEE J. Quantum Electron., vol. 49, No. 7, pp. 599-606, July 2013).
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.